Magpie.Auth.TokenProvider backed by a plain access token string.

This is what Magpie.Client.new("ACCESS_TOKEN") uses internally, so the two forms below are equivalent:

Magpie.Client.new("ACCESS_TOKEN")
Magpie.Client.new(token_provider: {Magpie.Auth.StaticToken, "ACCESS_TOKEN"})

There is nothing to refresh here: refresh_token/1 hands back the very same token, so a Dropbox expired_access_token error propagates to the caller as a regular Magpie.Error. Dropbox access tokens are short-lived (~4 hours) — use Magpie.Auth.TokenServer for anything long-running.